1. Field of the Invention
Exemplary aspects of the present invention generally relate to an image forming apparatus, such as a copier, a facsimile machine, a printer, or a multi-functional system including a combination of at least two of these functions.
2. Description of the Background Art
Conventionally, a generally known image forming apparatus, such as a copier, a facsimile machine, a printer, or a multi-functional system including a combination of at least two of these functions, can form an image on a sheet serving as a printable recording medium in various sizes, for example, A4, A3, A5, B4, and so forth. To do so, the image forming apparatus employs a fixing unit to fix the image on the sheet with heat and pressure. The fixing unit, which extends across the entire width of that recording medium which has the widest width that the image forming apparatus can accommodate, includes a heating member, a fixing member, and an auxiliary fixing member.
With different sizes of the recording sheet, such as A4 size and A5 size, which are not as wide as the A3 size (297 mm) sheets having the widest width that the image forming apparatus can accommodate and therefore do not extend across the entire width of the fixing unit, when the fixing unit fixes the image onto the A4 size or A5 size sheet, temperatures of the fixing unit at or near lateral end portions of the heating member, the fixing member, and the auxiliary fixing member over which the A4 size and A5 size recording media do not extend rise significantly, resulting in problems such as uneven fixation of the image, deterioration of the fixing member, and so forth, ultimately causing damage to the fixing unit.
In particular, in order to reduce a rise time of the image forming apparatus, that is, the time required for the image forming apparatus to start printing after power is turned on, a high-power heating member and a relatively thin-film fixing member have come to be required. However, a drawback to this configuration is that temperatures at the end portions of the heating member, the fixing member, and so forth rise significantly.
To address the above-described problems, one related-art fixing unit includes a regulator (hereinafter “regulator”) that changes how much of the fixing member is heated along its width (hereinafter also referred to as the heat width of the fixing member) depending on the width of the recording sheet being printed. This fixing unit employs an induction heating method using electromagnetic induction. In the induction heating method, a coil generates a magnetic field serving as a heat source.
The regulator consists of a heating portion and a regulating portion. Ferrite is used in the heating portion and copper used in the regulating portion of the regulator, thereby producing a difference in heating efficiency and thus regulating the heat width of the heating member.
Furthermore, the regulator rotates in association with the recording sheet so as to adjust the heat width of the heating member, thus changing the heat width of the fixing member to match the width of the recording sheet. As a result, a significant rise in temperatures at the end portions of the heating member, the fixing member, and so forth can be prevented.
However, there is a drawback to this configuration when printing is performed continuously on recording sheets of various sizes (for example, A4, A3, A5, B4) and different kinds (for example, an envelope, a paper sheet, etc.).
For example, when printing is performed alternately on an envelope and a different kind of a printable medium from the envelope such as a sheet of paper, or when printing is performed alternately in vertical and horizontal directions on a plurality of recording sheets of the same size fed from a plurality of sheet feeders (generally known as “rotary sorting”), printing is continuously performed on the recording sheets of different widths.
In order to continuously print an image on the recording sheets of different sizes having different widths, in other words, when the image forming apparatus that can accommodate an A3-size recording sheet continuously performs printing on the A3 recording sheet, then the A5 recording sheet, and then the A4 recording sheet sequentially, in that order, without changing the heat width of the fixing unit for each recording sheet for the sake of maintaining a certain print speed throughout the printing, either a significant rise in temperatures or insufficient heat at the end portions of the heating member, the fixing member, and so forth, occurs.
In this regard, the heat width needs to be changed for every sheet, necessitating extra time for changing the heat width. Consequently, after the first sheet is fed, a certain amount of time needs to be provided before the second sheet is fed, thereby reducing printing speed for continuous printing.
In view of the above, various approaches have been proposed in an attempt to solve the problem described above.
For example, in one conventional heating member, in order to prevent temperature rises in areas where the recording sheet does not contact the fixing member, a driving speed is switched from a normal speed to a low-speed mode only when a relatively small-size recording sheet is specifically designated for printing by the image forming apparatus, or, if the apparatus is part of a network, by a host computer (server) that controls printing. By contrast, when there is no designation of the small-size recording sheet, the driving speed is set at a normal speed.
Thus, even when a detector detects an actual recording sheet being a small size after the device starts to be driven at the normal speed, the driving speed is still not switched to the low-speed mode for the small sized paper, and throughput control is performed corresponding to the small sized recording sheet at the normal speed.
With this configuration, no detector for detecting the size of the recording sheet is necessitated at an opening from which the recording sheet is fed, thereby providing an inexpensive configuration that still allows the low-speed mode to be set easily. Furthermore, under any conditions, the fixing device does not fail due to temperature rises where the recording sheet is not present.
However, there is also a drawback to the foregoing approach in that when the small-sized recording sheet is designated by the user, switching the driving speed to the low-speed mode, the printing speed for continuous printing is forced to slow down.
Furthermore, when switching the driving speed from the normal-speed mode to the low-speed mode, extra time is required for switching the mode, thereby also slowing down the printing speed for continuous printing.
In another related-art approach to prevent reduction of the printing speed for the first page to be printed, both a relatively large heater as well as a small heater are provided in the image forming apparatus and heating is switched between the two according to paper size. In the image forming apparatus of this approach, when fixing the first sheet after printing is initiated, a CPU for controlling operation of the image forming apparatus always supplies power to the large heater regardless of the paper size.
After the small-sized recording sheet is fixed, the large heater is turned on. Furthermore, after the small-sized recording sheet is fixed, the CPU not only turns on the large heater, but also measures the temperature of the heater and stops transport of the recording sheet when the temperature of the heater does not reach a predetermined temperature. Fixing is resumed when the temperature of the heater reaches the predetermined temperature.
A drawback to the foregoing configuration is that, because the large heater is turned on before the first printing and after the small-sized recording sheet is printed, a significant rise in temperatures occurs at the end portions of the fixing member, the auxiliary fixing member, the heat source, and so forth. Moreover, when printing is alternately performed between the large-sized paper and the small-sized paper, problems such as uneven fixation and deterioration of the fixing member occur.
Furthermore, according to the foregoing technology, two heaters, both the large-sized heater and the small-sized heater, are necessitated, thereby increasing the cost of the image forming apparatus as a whole. In addition, generally, in order to maintain imaging quality, the fixing unit is periodically replaced, causing the cost per paper to increase despite the effort of improving the printing speed for continuous printing.
In order to overcome such problems, when printing out a plurality of the recording sheets, the recording sheets are divided into different groups by width, and the group to which the preceding recording sheet belongs is compared with the group to which the subsequent recording sheet belongs.
Only when the amount of heat generated in the vicinity of the end portions of the fixing member, the auxiliary fixing member, the heat source, and so forth is not sufficient, a relatively long interval is provided before subsequent transport of transfer paper is started. Other than that, continuous printing is performed at predetermined intervals. Accordingly, a certain printing speed can be maintained when continuously printing, while a significant rise in temperatures and/or insufficient heating can be prevented.
However, there is also a drawback to this approach. Since the recording sheet in the preceding group and the recording sheet in the subsequent group are compared with each other, and the relatively long interval is provided before the subsequent transport of the recording sheet is started, the timing with which the recording sheet is transported varies in each case, thus decreasing overall printing speed.